What happened
On May 9, 2011, a Cessna U206G, registration OO-A\* , was conducting a private flight from Mbandaka to Basankusu airfield in the Democratic Republic of Congo. While the aircraft was entering the right downwind circuit for runway 24, the Teledyne Continental Motors IO-520-F engine suddenly lost all power. Within approximately 10 seconds, the propeller ceased rotation.
Despite the sudden loss of power, the pilot managed a steep descent and diverted to runway 06. The aircraft touched down safely between 50 and 100 meters from the threshold and came to a complete stop at the end of the runway. There were no injuries among the five occupants, and the airframe sustained no damage.
The investigation
Following the incident, the engine was transported to Belgium for a detailed teardown. Investigators examined the internal components of the crankcase and the condition of the bearings. The inspection revealed that the crankshaft had fractured at cheek number 3.
Further analysis showed that the main journal bearings number 2 and 3 had rotated within the crankcase, causing extreme wear to the housing. The investigation also identified that the connecting rod bearing number 2 had suffered significant overheating and damage. Upon inspecting the mating surfaces of the crankcase halves, investigators found traces of a white silicone-based sealant. This was contrasted with the required maintenance procedure, which specifies the use of Permatex Aviation Grade 3 and silk thread to ensure an oil-tight seal and proper clamping force.
Findings
- The engine failure was caused by a fracture of the crankshaft.
- The root cause was the use of an unapproved sealant on the parting surfaces of the crankcase halves during a previous overhaul.
- The application of this sealant, particularly around the through bolts, prevented the necessary torque and clamping force from being maintained.
- This improper assembly led to the fretting of the crankcase halves and the subsequent rotation of the main bearings.
- The shifting bearings caused the crankshaft to lose adequate support, leading to vibration, deformation, and eventual structural failure of the crankshaft.